New tank question

I know a lot of people use racks or tubs for their snakes, but I prefer to have them visible. I'm moving my bci (3.5-4ft? *just a guess, never been measured) from a standard 20gal to a 40 long. Looks plenty big enough for him for now. I was going to have a custom enclosure built but it's a long story and it never got done. I bought the 40 because I would've bought it for my bp eventually anyway so this buys me some time to have the 5x2x2 built.

My question is, at what size is he too large for the 40long?

Basic rule of thumb is the length should be no less then 50% the length of the snake, so technically the 40 Gallon Long is I think 3 feet so it should be "ok" up to 72 inches but with a boa they are bulky so I would not want took keep him in there much past 5 - 5.5 feet.

If you are feeding moderately and your snake is a male (which seems to be the case) then I would say you will get a year or better with this tank as a go between.

That is just a guess so many things effect growth rates,

Yes, he's a male. At roughly 9 months old he's about 3.5ft. Like I said, I've never measured him. I'm just guessing, comparing his appearance to that of my 3ft ball, which doesn't really help at all, does it? lol.

He gets 1 small rat a week, with no meals during shed.